1. Field of the Invention
The present invention relates to an inverter transformer for lighting a discharge lamp as a light source of backlight device for a liquid crystal display apparatus, and particularly to an inverter transformer to provide a plurality of outputs for lighting a plurality of discharge lamps.
2. Description of the Related Art
A liquid crystal display (LCD) apparatus, which is used for electronic appliances, such as a television, a personal computer, and the like, does not emit light by itself, and therefore a lighting system, such as a backlight device, is required. A discharge lamp is used as a light source for such a backlight device, and a cold cathode fluorescent lamp (CCFL) is typically employed as a discharge lamp. Recently, the screen size of an LCD apparatus, for example, an LCD television, is becoming larger and larger, and a plurality of CCFLs are used in order to achieve the high brightness level required. A high voltage is required for lighting a CCFL, and a high frequency voltage generated at a switching portion of an inverter circuit is boosted by an inverter transformer up to a high voltage required for lighting a CCFL.
A typical conventional inverter transformer provides a single output, and for lighting a plurality of CCFLs, inverter transformers must be provided in a number equal to the number of the CCFLs used. Accordingly, a large size LCD apparatus requires a number of inverter transformers thus increasing the size of a backlight device. To deal with this size increase issue, an inverter transformer is disclosed which includes a plurality of secondary windings to thereby provide a plurality of outputs (refer, for example, to Patent Document 1).
FIG. 11 shows such an inverter transformer as disclosed in Patent Document 1. Referring to FIG. 11, an inverter transformer 120 includes a frame magnetic core 121 shaped rectangular, and three I-cores 123a, 123b and 123c arranged inside the frame magnetic core 121. The I-cores 123a, 123b and 123c respectively have primary windings 124a, 124b and 124c and secondary windings 125a, 125b and 125c wound therearound thereby enabling three CCFLs to be lit. In the inverter transformer 120, voltages with an identical polarity are induced at the secondary windings 125 (125a/125b/125c) by a current flowing in the primary windings 124 (124a/124b/124c), and hence no voltage difference exists at the secondary windings 125 thus allowing the withstand voltage to be lowered, which results in downsizing of the inverter transformer 120.
With an increase of an LCD apparatus and a resultant increase of a backlight device, the length dimension of a CCFL as a light source is inevitably increased. A higher voltage is required for starting to light a CCFL with an increased length, and the output voltage at the secondary winding becomes higher thus requiring an increased withstand voltage. Also, in a common connection structure where the low voltage side of the CCFL is provided with a return line, the brightness at the low voltage side of the CCFL tends to easily go down. Further, since a number of wiring materials of a high withstand voltage are required, problems are raised about safety and cost.
To overcome the lowering of the brightness at the low voltage side and to reduce the number of wiring materials of a high withstand voltage, various approaches have been proposed where CCFLs are driven with a double voltage. For example, reverse polarity high voltages having their phases shifted from each other by 180 degrees (opposite phase) are applied respectively to both terminals of a long CCFL or a bent lamp such as a U-shape lamp, or to two CCFLs which have their respective low voltage sides connected to each other. In the approaches described above, in order to apply a reverse polarity high voltage to both terminals of a CCFL, an inverter transformer includes secondary windings to generate high AC voltages independent of each other, and the secondary windings are wound in opposite directions so that the output voltages have their phases shifted from each other by 180 degrees (refer, for example, to Patent Document 2).
FIG. 12 is a top plan view of an inverter transformer disclosed in Patent Document 2, and FIG. 13 is an exploded perspective view of magnetic cores of the inverter transformer of FIG. 12.
An inverter transformer shown in FIG. 12 includes a primary winding 230, and two primary windings 240a and 240b magnetically coupled to the primary winding 230. Further included in the inverter transformer are magnetic cores 250 and 260 shown in FIG. 13, which are made of a magnetic material. Referring to FIG. 13, the magnetic core 25 includes a rectangular support 251, two columnar supports 252 and 253, and an elongated projection 254 disposed along the length of the rectangular support 251 and sandwiched between the rectangular support 251 and the columnar supports 252 and 253. A cutout 255 is formed between the two columnar supports 252 and 253 which are to be inserted respectively in the centers of the secondary windings 240a and 240b, and a cutout 265 is formed at the magnetic core 260. The magnetic coupling between the secondary windings 240a and 240b is caused to weaken due to the cutouts 255 and 265, thus preventing the interference of the magnetic fluxes flowing through the columnar supports 252 and 253. And, since the primary windings 240a and 240b are wound in opposite directions with the same turn number, reverse polarity voltages are outputted respectively at the primary windings 240a and 240b.     Patent Document 1: Japanese Patent Application Laid-Open No. 2002-353044    Patent Document 2: Japanese Patent Application Laid-Open No. 2001-148318